Volkov states are exact solutions of the Dirac equation in the presence of an arbitrary plane wave. Accounting for the interaction between the Dirac field and the electromagnetic field, however, Volkov states, as well as free photon states, are not stable in the presence of the background plane-wave field but “decay” as electrons/positrons can emit photons and photons can transform into electron-positron pairs. By using the solutions of the corresponding Schwinger-Dyson equations within the locally constant field approximation, we compute the probabilities of nonlinear single Compton scattering and nonlinear Breit-Wheeler pair production by including the effects of the decay of electron, positron, and photon states. As a result, we find that the probabilities of these processes can be expressed as the integral over the light-cone time of the known probabilities valid for stable states per unit of light-cone time times a light-cone time-dependent exponential damping function for each interacting particle. The exponential function for an incoming (outgoing) either electron/positron or photon at each light-cone time corresponds to the total probability that either the electron/positron emits a photon via nonlinear Compton scattering or the photon transforms into an electron-positron pair via nonlinear Breit-Wheeler pair production until that light-cone time (from that light-cone time on). It is interesting that the exponential damping terms depend not only on the particles momentum but also on their spin (for electrons/positrons) and polarization (for photons). This additional dependence on the discrete quantum numbers prevents the application of the standard electron/positron spin and photon polarization sum rules in computing, for example, total probabilities.

Inspired by recent progress on classical limits of scattering amplitudes, we show that hard thermal loops can be obtained from classical limits of off-shell currents. The classicality of hard thermal loops is made manifest by associating classical wavenumbers to soft particles. We compute the classical limit of these currents in QED, QCD, and gravity. Our proposal does not involve the introduction of ghosts.

The nonanalyticity and the sign problem in the $Z_3$-symmetric heavy quark model at low temperature are studied phenomenologically. For the free heavy quarks, the nonanalyticity is analyzed in relation to the zeros of the grand canonical partition function. The $Z_3$-symmetric effective Polyakov-line model (EPLM) in the strong coupling limit is also considered as an phenomenological model of $Z_3$-symmetric QCD with large quark mass at low temperature. We examine how the $Z_3$-symmetric EPLM approaches the original one in the zero-temperature limit. The effects of the $Z_3$ symmetry affect the structure of zeros of the microscopic probability density function at the nonanalytic point. The average value of the Polyakov line can detect the structure, while the other thermodynamic quantities are not sensitive to the structure in the zero-temperature limit. The effect of the imaginary quark chemical potential is also discussed. The imaginary part of the quark number density is very sensitive to the symmetry structure at the nonanalytical point. For a particular value of the imaginary quark number chemical potential, large quark numbers may be induced in the vicinity of the nonanalytical point.

Compact objects inspiraling into supermassive black holes, known as extreme-mass-ratio inspirals, are an important source for future space-borne gravitational-wave detectors. When constructing waveform templates, usually the adiabatic approximation is employed to treat the compact object as a test particle for a short duration, and the radiation reaction is reflected in the changes of the constants of motion. However, the mass of the compact object should have contributions to the background. In the present paper, employing the effective-one-body formalism, we analytically calculate the trajectories of a compact object around a massive Kerr black hole with generally three-dimensional orbits and express the fundamental orbital frequencies in explicit forms. In addition, by constructing an approximate “constant” similar to the Carter constant, we transfer the dynamical quantities such as energy, angular momentum, and the “Carter constant” to the semilatus rectum, eccentricity, and orbital inclination with mass-ratio corrections. The linear mass-ratio terms in the formalism may not be sufficient for accurate waveforms, but our analytical method for solving the equations of motion could be useful in various approaches to building waveform models.

We present a definition for a pair of null coordinates that are naturally adapted to the horizons and future null infinity of Kerr spacetime, and that are generated by the center-of-mass sections at future null infinity. They are a smooth, round family of null hypersurfaces which foliate the Kerr spacetime in an outgoing and an ingoing sense, respectively, and they have a regular extension across the horizons. Because of Kerr’s peculiar geometry, the construction involves a nonlinear differential equation for a scalar function related to Carter’s constant, whose solution cannot be expressed in terms of simple analytic functions. We present the numerical solution of this scalar for a particular choice of the geometrical parameters. In this setting, there naturally appears a two-dimensional spacelike family of round surfaces $S_{r_s}$ that are parametrized by $r_s$, which are the intersections of both null coordinates, where $r_s$ can be thought of as the tortoise coordinate extension for the Kerr spacetime. The $S_{r_s}$ surfaces are axially symmetric, but they have an $(r,\theta )$ dependence in Boyer-Lindquist coordinates. They can also be characterized in a complete geometrical way by their Gaussian and extrinsic curvature scalars, which we were able to compute by the use of Geroch-Held-Penrose formalism. We compare our definition with other previous attempts in the literature, and we show that all of them have divergent behavior at the axis of symmetry. Thus, our construction presents the first double null coordinate system which makes possible computations over all of the Kerr spacetime.

We study the quasi-periodic oscillations from the accretion disk around rotating traversable wormholes by means of the resonance models. We investigate the linear stability of the circular geodesic orbits in the equatorial plane for a general class of wormhole geometries deriving analytical expressions for the epicyclic frequencies. Since wormholes can often mimic black holes in the astrophysical observations, we analyze the properties of the quasi-circular oscillatory motion in comparison with the Kerr black hole. We demonstrate that wormholes possess distinctive features, which can be observationally significant. It is characteristic for the Kerr black hole that the orbital and the epicyclic frequencies obey a constant ordering in the whole range of the spin parameter. In contrast, for wormhole spacetimes we can have various types of orderings between the frequencies in the different regions of the parametric space. This enables the excitation of much more diverse types of resonances including parametric and forced resonances of lower order, which could lead to stronger observable signals. In addition, for corotating orbits the resonances can be excited in a very close neighborhood of the wormhole throat for a wide range of values of the angular momentum, making wormholes a valuable laboratory for testing strong gravity.

The Belinski-Khalatnikov-Lifshitz (BKL) conjecture predicts a chaotic alternation of Kasner epochs in the evolution of generic classical spacetimes towards a spacelike singularity. As a first step towards understanding the full quantum BKL scenario, we analyze a vacuum Bianchi II model with local rotational symmetry, which presents just one Kasner transition. During the Kasner epochs, the quantum state is coherent, and it is thus characterized by constant values of the different quantum fluctuations, correlations, and higher-order moments. By computing the constants of motion of the system, we provide, for any peaked semiclassical state, the explicit analytical transition rules that relate the parametrization of the asymptotic coherent state before and after the transition. In particular, we obtain the modification of the transition rules for the classical variables due to quantum backreaction effects. This analysis is performed by considering a high-order truncation in moments (the full computations are performed up to fifth-order, which corresponds to neglecting terms of an order ${\hslash }^3$), providing a solid estimate about the quantum modifications to the classical model. Finally, in order to understand the dynamics of the state during the transition, we perform some numerical simulations for an initial Gaussian state, that show that the initial and final equilibrium values of the quantum variables are connected by strong and rapid oscillations.

The possibility to detect gravitational waves (GW) from planetary-mass primordial black hole (PBH) binaries with electromagnetic (EM) detectors of high-frequency GWs is investigated. We consider two patented experimental designs, based on the inverse Gertsenshtein effect, in which incoming GWs passing through a static magnetic field induce EM excitations inside either a TM cavity or a TEM waveguide. The frequency response of the detectors is computed for post-Newtonian GW waveforms. We find that such EM detectors based on current technology may achieve a strain sensitivity down to $h\sim 10^{-30}$, which generates an EM induced power of $10^{-10}\mathrm{W}$. This allows the detection of PBH binary mergers of mass around $10^{-5}{M}_{\odot }$ if they constitute more than 0.01 percent of the dark matter, as suggested by recent microlensing observations. We envision that this class of detectors could also be used to detect cosmological GW backgrounds and probe sources in the early Universe at energies up to the grand unified theory scale.

We investigate the $H_0$ tension in a range of extended model frameworks beyond the standard $\mathrm{\Lambda }\mathrm{CDM}$ without the data from the cosmic microwave background (CMB). Specifically, we adopt the data from baryon acoustic oscillations, big bang nucleosynthesis, and type Ia supernovae as indirect measurements of $H_0$ to study the tension. We show that the estimated value of $H_0$ from indirect measurements is overall lower than that from direct local ones regardless of the data sets and a range of extended models to be analyzed, which indicates that, although the significance of the tension varies depending on models, the $H_0$ tension persists in a broad framework beyond the standard $\mathrm{\Lambda }\mathrm{CDM}$ model even without CMB data.

In this paper, we calculate the next-to-leading order (NLO) quantum chromodynamics (QCD) corrections to the exclusive processes $B_c^{+}\to {\chi }_{cJ}(h_c){\pi }^{+}$ in the framework of the nonrelativistic QCD (NRQCD) factorization formalism. The results show that NLO QCD corrections markedly enhance the branching ratios with $K$ factors of about 2.5. In combination with the study of $B_c^{+}\to J/\psi {\pi }^{+}$, we find that the NLO NRQCD prediction for the ratio of branching fractions $\frac{\mathcal{B}(B_c^{+}\to {\chi }_{c0}{\pi }^{+})}{\mathcal{B}(B_c^{+}\to J/\psi {\pi }^{+})}$ is then compatible with the experimental measurement.

An extended collinearly improved Balitsky-Kovchegov evolution equation in the target rapidity representation is derived by including the running coupling corrections during the expansion of the “real” $S$-matrix. We find that the running coupling brings important modifications to the evolution equation, as one can see that there are extra contributions to the evolution kernel once the running coupling is included. To identify the significance of the modifications, we numerically solve the evolution equation with and without the running coupling contributions during the $S$-matrix expansion. The numerical results show that the scattering amplitude is largely suppressed by the running coupling corrections, which indicate that one needs to consider the running coupling contributions during the derivation of the nonlinear evolution equation in the target rapidity representation.

Recently, the so-called relative transverse activity classifier, $R_{\mathrm{T}}$, has been proposed as a tool to disentangle the particle production originated from the soft and hard QCD processes in proton-proton (pp) collisions. $R_{\mathrm{T}}$ is a useful quantity to study particle production in events with exceptionally large or small activity in the transverse region with respect to the event-averaged mean. Contrary to the expectations, the preliminary results of the ALICE Collaboration indicate that, e.g., the proton-to-pion ratio does not exhibit the characteristic enhancement at intermediate $p_{\mathrm{T}}$ in events with large $R_{\mathrm{T}}$ with respect to minimum-bias pp collisions. In this work, we investigate the origin of this effect using the pythia 8 and herwig 7 Monte Carlo event generators. The effect is a consequence of a selection bias attributed to wide-angle gluon emissions which creates jets that populate the transverse region. Therefore, we propose a modified version of $R_{\mathrm{T}}$ in order to suppress its sensitivity to hard gluon Bremsstrahlung, and enhance the sensitivity to soft Multiparton Interactions (MPI). This approach could be useful in order to study the particle production in the jetlike region as a function of MPI. The implementation of these ideas in data will provide more insight into the production mechanisms of hadrons in high-multiplicity pp collisions, and its connection with heavy-ion phenomena.

We perform a comprehensive new Monte Carlo analysis of high-energy lepton-lepton, lepton-hadron and hadron-hadron scattering data to simultaneously determine parton distribution functions (PDFs) in the proton and parton to hadron fragmentation functions (FFs). The analysis includes all available semi-inclusive deep-inelastic scattering and single-inclusive $e^{+}{e}^{-}$ annihilation data for pions, kaons and unidentified charged hadrons, which allows the flavor dependence of the fragmentation functions to be constrained. Employing a new multistep fitting strategy and more flexible parametrizations for both PDFs and FFs, we assess the impact of different datasets on sea quark densities and confirm the previously observed suppression of the strange quark distribution. The new fit, which we refer to as “JAM20-SIDIS,” will allow for improved studies of universality of parton correlation functions, including transverse momentum dependent (TMD) distributions, across a wide variety of process, and the matching of collinear to TMD factorization descriptions.

A modest extension of the Standard Model by two additional Higgs doublets—the Higgs troika model—can provide a well-motivated scenario for successful baryogenesis if neutrinos are Dirac fermions. Adapting the “spontaneous flavor violation” framework, we consider a version of the troika model where light quarks have significant couplings to the new multi-TeV Higgs states. Resonant production of new scalars leading to dijet or top-pair signals are typical predictions of this setup. The initial- and final-state quarks relevant to the collider phenomenology also play a key role in baryogenesis, potentially providing direct access to the relevant early-Universe physics in high-energy experiments. Viable baryogenesis generally prefers some hierarchy of masses between the observed and the postulated Higgs states. We show that there is a complementarity between direct searches at a future 100 TeV $pp$ collider and indirect searches at flavor experiments, with both sensitive to different regions of parameter space relevant for baryogenesis. In particular, measurements of $D-\overline{D}$ mixing at LHCb probe much of the interesting parameter space. Direct and indirect searches can uncover the new Higgs states up to masses of $\mathcal{O}(10)\mathrm{TeV}$, thereby providing an impressive reach to investigate this model.

We present a reanalysis of nucleon decay in the context of the $R$-parity violating Minimal Supersymmetric Standard Model, updating bounds on $R$-parity violating parameters against recent experimental and lattice results. We pay particular attention to the derivation of these constraints and specifically to the hadronic matrix elements, which usually stand as the limiting factor in order to derive reliable bounds, except for these few channels that have been studied on the lattice.

This paper presents a novel technique for mitigating electrode backgrounds that limit the sensitivity of searches for low-mass dark matter (DM) using xenon time projection chambers. In the Large Underground Xenon (LUX) detector, signatures of low-mass DM interactions would be very low-energy ($\sim \mathrm{keV}$) scatters in the active target that ionize only a few xenon atoms and seldom produce detectable scintillation signals. In this regime, extra precaution is required to reject a complex set of low-energy electron backgrounds that have long been observed in this class of detector. Noticing backgrounds from the wire grid electrodes near the top and bottom of the active target are particularly pernicious, we develop a machine learning technique based on ionization pulse shape to identify and reject these events. We demonstrate the technique can improve Poisson limits on low-mass DM interactions by a factor of 1.7–3 with improvement depending heavily on the size of ionization signals. We use the technique on events in an effective 5 tonne·day exposure from LUX’s 2013 science operation to place strong limits on low-mass DM particles with masses in the range $m_{\chi }\in 0.15–10\mathrm{GeV}$. This machine learning technique is expected to be useful for near-future experiments, such as LUX-ZEPLIN and XENONnT, which hope to perform low-mass DM searches with the stringent background control necessary to make a discovery.

The mismatch in the value of the Hubble constant from low- and high-redshift observations may be recast as a discrepancy between the low- and high-redshift determinations of the luminosity of Type Ia supernovae, the latter featuring an absolute magnitude which is $\approx 0.2\mathrm{mag}$ lower. Here, we propose that a rapid transition in the value of the relative effective gravitational constant ${\mu }_G\equiv \frac{G_{\mathrm{eff}}}{G_N}$ at $z_t\simeq 0.01$ could explain the lower luminosity (higher magnitude) of local supernovae, thus solving the $H_0$ crisis. In other words, here the tension is solved by featuring a transition at the perturbative rather than background level. A model that features ${\mu }_G=1$ for $z\lesssim 0.01$ but ${\mu }_G\simeq 0.9$ for $z\gtrsim 0.01$ is trivially consistent with local gravitational constraints but would raise the Chandrasekhar mass and so decrease the absolute magnitude of type Ia supernovae at $z\gtrsim 0.01$ by the required value of $\approx 0.2\mathrm{mag}$. Such a rapid transition of the effective gravitational constant would not only resolve the Hubble tension but it would also help resolve the growth tension as it would reduce the growth of density perturbations without affecting the $\mathrm{Planck}/\mathrm{\Lambda }\mathrm{CDM}$ background expansion.

We consider the scattering of an x-ray free-electron laser (XFEL) beam on the superposition of a strong magnetic field ${\mathbf{B}}_{\mathrm{ext}}$ with the Coulomb field ${\mathbf{E}}_{\mathrm{ext}}$ of a nucleus with charge number $Z$. In contrast to Delbrück scattering (Coulomb field only), the magnetic field ${\mathbf{B}}_{\mathrm{ext}}$ introduces an asymmetry (i.e., polarization dependence) and renders the effective interaction volume quite large, while the nuclear Coulomb field facilitates a significant momentum transfer $\mathrm{\Delta }\mathbf{k}$. For a field strength of $B_{\mathrm{ext}}={10}^6\mathrm{T}$ (corresponding to an intensity of order ${10}^{22}\mathrm{W}/{\mathrm{cm}}^2$) and an XFEL frequency of 24 keV, we find a differential cross section $d\sigma /d\mathrm{\Omega }\sim {10}^{-25}{Z}^2/(\mathrm{\Delta }\mathbf{k}{)}^2$ in forward direction for one nucleus. Thus, this effect might be observable in the near future at facilities such as the Helmholtz International Beamline for Extreme Fields at the European XFEL.

Theories beyond the standard model involving a sub-GeV-scale vector $Z_d$ mediator have been largely studied as a possible explanation of the experimental values of the muon and electron anomalous magnetic moments. Motivated by the recent determination of the anomalous muon magnetic moment performed at Fermilab, we derive the constraints on such a model obtained from the magnetic moment determinations and the measurements of the proton and cesium weak charge, $Q_W$, performed at low-energy transfer. In order to do so, we revisit the determination of the cesium $Q_W$ from the atomic parity violation experiment, which depends critically on the value of the average neutron rms radius of ${}^{133}\mathrm{Cs}$, by determining the latter from a practically model-independent extrapolation from the recent average neutron rms radius of ${}^{208}\mathrm{Pb}$ performed by the PREX-2 Collaboration. From a combined fit of all the aforementioned experimental results, we obtain rather precise limits on the mass and the kinetic mixing parameter of the $Z_d$ boson, namely $m_{Z_d}={47}_{-16}^{+61}\mathrm{MeV}$ and $ϵ={2.3}_{-0.4}^{+1.1}\times {10}^{-3}$, when marginalizing over the $Z-Z_d$ mass mixing parameter $\delta$.

We discuss aspects of generic two-dimensional dilaton gravity theories. The two-dimensional geometry is in general conformal to the two-dimensional anti–de Sitter space (${\mathrm{AdS}}_2$) and has IR curvature singularities at zero temperature; this can be regulated by a black hole. The on-shell action is divergent; we discuss the holographic energy-momentum tensor by adding appropriate counterterms. For theories obtained by dimensional reduction of the gravitational sector of higher dimensional theories, for instance higher dimensional AdS gravity as a concrete example, the two-dimensional description dovetails with the higher dimensional one. We also discuss more general theories containing an extra scalar field which now drives nontrivial dynamics. Finally we discuss aspects of the two-dimensional cosmological singularities discussed in earlier work. These studies suggest that generic two-dimensional dilaton gravity theories are somewhat distinct from Jackiw-Teitelboim (JT) gravity and theories “near JT”.